The present invention relates to a method and apparatus for controlling the air-fuel ratio of internal combustion engines, or more in particular to a method and apparatus for air-fuel ratio control in which the air-fuel ratio is controlled to an optimum value associated with the optimum fuel consumption rate by feedback control.
Generally, the air-fuel ratio is set to a stoichiometric ratio or a leaner value than that with emphasis placed on the fuel consumption rate under general running conditions, that is, to about 13 or a value with the highest output while the acceleration pedal is depressed to the full such as when ascending a slope, and to a value considering the stability when idling.
In the conventional air-fuel control under general running conditions, the carburetor is subjected to open-loop control and some loss of the fuel consumption rate is caused by variations between internal combustion engines, the secular variation of the internal combustion engine involved and variations between carburetors. An electronically-controlled fuel injection system for measuring the intake air amount of the internal combustion engine with an air flow sensor or the like, computing the required fuel amount with a computer or the like and injecting the fuel from fuel injectors according to the computation practically uses a closed loop control for deciding the direction of the stoichiometric ratio (about 15) from the oxygen sensor provided in the exhaust pipe and for correcting the fuel amount. Also, a closed loop control for the carburetor in which the air amount of the air bleed is corrected by determining the direction of the stoichiometric ratio by the oxygen sensor finds partial applications. These closed loop controls are capable of correcting the variations of the air-fuel ratio, but result in the loss of fuel consumption rate since the stoichiometric ratio is not a value associated with the best fuel consumption rate.
A conventional method has been suggested for controlling the fuel consumption rate without the above-mentioned loss. In such a control method, the air bypassing an air amount sensor and the throttle valve is made to dither at regular intervals of time between rich and lean sides of the air-fuel ratio, the direction of the air-fuel ratio associated with an improved fuel consumption rate is determined, and the air-fuel ratio is corrected by an auxiliary air valve bypassing the air amount sensor. In this method, the engine is run once at each of the relatively rich and lean levels of the air-fuel ratio, so that the engine speed Ner for the rich air-fuel ratio is compared with the engine speed Nel for the lean air-fuel ratio, and if Ner is larger than Nel, the bypass air amount is reduced, while if Ner is smaller than Nel, the bypass air amount is increased.
In determining the change of output from the engine speed which is changed by various factors, however, the above-mentioned conventional method of control is incapable of determining whether the engine speed is changed by the change of the air-fuel ratio or operation of the acceleration pedal or by ascending or descending a slope, with the result that the control may be effected in the direction reverse to the improvement of fuel consumption rate, thus deteriorating the fuel consumption rate. Further, the air passing through the air amount sensor may change and also may not change in cases when the air is applied through a bypass of the air amount sensor and the throttle valve and when the air is not applied therethrough, and it could not be assumed that a fuel flow rate is always constant. As a result, it may occur that the best fuel consumption rate is not achieved but a loss is caused.